In the past, while various proposals have been made in relation to a control device for controlling the displacement of a fluid pressure actuator such as the length of a hydraulic cylinder, for example, a bucket leveler device has been described in Patent Document 1.
In a shovel loader or the like which comprises a boom which is rotated by a boom cylinder upwards and downwards with respect to a vehicle body, and a bucket which is fitted to the end portion of the boom, and which is tilted by a tilt cylinder, the above described bucket leveler device is provided with a bucket angle detector and a boom angle detector; and it decides, from the output signals of the bucket angle detector and the boom angle detector, that the bucket absolute angle (its angle relative to the ground surface) has become an angle which has been set, and it returns a bucket actuation lever to its neutral position when the bucket absolute angle is equal to the set angle. Furthermore, when the actual bucket absolute angle has changed from the set angle due to rotation of the boom, it calculates a bucket angle compensation signal according to the amount of this variation, and operates an electromagnetic valve with this bucket angle compensation signal, thus supplying pressure oil to a bucket cylinder so as to bring about the target bucket set angle; and thus it maintains the bucket angle at the set angle by varying its length.
Patent Document 1: Japanese Patent Laid-Open Publication Heisei 1-182419 (pages 3 and 4, FIG. 1).